Compounds with the Yb 21 Mn 4 Sb 18 structure show local disorder as shown in the PDF model (left) and Cd solid solution provides the highest zT (right).
We report a method to fabricate polymer microstructures with local control over the molecular orientation. Alignment control is achieved on molecular level in a structure of arbitrary form that can be from 1 to 100 μm in size, by fixing the local boundary conditions with micro-grating patterns. The method makes use of two-photon polymerization (Direct Laser Writing) and is demonstrated specifically in liquid-crystalline elastomers. This concept allows for the realization of free-form polymeric structures with multiple functionalities which are not possible to realize with existing techniques and which can be locally controlled by light in the micrometer scale.
Tetragonal tungsten bronzes Nb8−xW9+xO47−δallow a continuous variation of the charge carrier concentration while fulfilling the concept of a “phonon-glass electron-crystal” through intrinsic nanostructure.
Abstract Solid state reactions are notoriously slow, because the rate‐limiting step is diffusion of atoms or ions through reactant, intermediate, and product crystalline phases. This requires days or even weeks of high temperature treatment, consuming large amounts of energy. Metal oxides are particularly difficult to react, because they have high melting points. The study reports a high‐speed solid state fluorination of WO 3 with Teflon to the oxyfluorides WO 3– x F x on a minute (<10 min) scale by spark plasma sintering, a technique that is used typically for a high‐speed consolidation of powders. Automated electron diffraction analysis reveals an orthorhombic ReO 3 ‐type structure of WO 3– x F x with F atom disorder as demonstrated by 19 F magic angle spinning nuclear magnetic resonance spectroscopy. The potential of this new approach is demonstrated by the following results. i) Mixed‐ valent tungsten oxide fluorides WO 3– x F x with high F content (0 < x < 0.65) are obtained as metastable products in copious amounts within minutes. ii) The spark plasma sintering technique yields WO 3– x F x nanoparticles with high photocatalytic activity, whereas the corresponding bulk phases obtained by conventional solid‐state (ampoule) reactions have no photocatalytic activity. iii) The catalytic activity is caused by the microstructure originating from the processing by spark plasma sintering.
Energy savings motivated by economic and environmental reasons have brought attention to waste heat recovery technologies. Thermoelectric generators, previously employed as electric power generators in harsh conditions such as the found in space missions and remote ground locations, have arisen as an option to recover waste energy. Thermoelectric generators convert heat into electric power, but the low conversion efficiency of thermoelectric materials involves a challenge in the integration of thermoelectric generators to heat sources, as more energy losses than savings can be caused. As a result, the design process must be tailored to the system from which energy is to be recovered. This work fills the gap in previous reviewed literature, providing considerations focused on thermal management of heat sources for the design of thermoelectric generators and methods to evaluate specific energy sources and prototypes are presented. In addition, several applications were reviewed and main opportunities, advantages and drawbacks in different sectors are presented.
Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low‐cost materials with reasonable thermoelectric performances and cost‐efficient preparation techniques are required. In this context metal oxides are an interesting class of materials because of their inherent high‐temperature stability and relative high sustainability. Their thermoelectric performance, however, needs to be improved for wide application. Compounds with adaptive structures are a very interesting class of materials. A slight reduction of early transition metal oxides generates electrons as charge carriers and crystallographic shear planes as structure motif. The crystallographic shear planes lead to a reduction of intrinsic thermal conductivity. At the same time, the electronic transport properties can be tuned by the degree of reduction. So far only a few transition metal oxides with adaptive structures have been investigated with respect to their thermoelectric properties, leaving much room for improvement. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties.
